15 Jul 2021

15 Jul 2021

Review status: this preprint is currently under review for the journal ACP.

The role of anthropogenic aerosols in the anomalous cooling from 1960 to 1990 in the CMIP6 Earth System Models

Jie Zhang1, Kalli Furtado2, Steven T. Turnock2, Jane P. Mulcahy2, Laura J. Wilcox3, Ben B. Booth2, David Sexton2, Tongwen Wu1, Fang Zhang1, and Qianxia Liu1 Jie Zhang et al.
  • 1Beijing Climate Center, China Meteorological Administration, Beijing, China,100081
  • 2Met Office Hadley Centre, Exeter, UK, EX1 3PB
  • 3National Centre for Atmospheric Science, Department of Meteorology, University of Reading, Reading, UK

Abstract. The Earth System Models (ESMs) that participated in the 6th Coupled Model Intercomparison Project (CMIP6) tend to simulate excessive cooling in surface air temperature (TAS) between 1960 and 1990. The anomalous cooling is pronounced over the Northern Hemisphere (NH) midlatitudes, coinciding with the rapid growth of anthropogenic sulfur dioxide (SO2) emissions, the primary precursor of atmospheric sulphate aerosols. Historical simulations with and without anthropogenic aerosol emissions indicate that the anomalous cooling within the ESMs is potentially due to in part from overestimated anthropogenic aerosols and the enhanced aerosol-forcing-sensitivity. Structural uncertainties between ESMs that contribute to these two factors have a larger impact on the anomalous cooling than internal variability. CMIP6 simulations can also help us to quantify the relative contributions of aerosol-forcing-sensitivity by aerosol-radiation interactions (ARI) and by aerosol-cloud interactions (ACI). However, even when the aerosol-forcing-sensitivity is similar between ESMs, the relative contributions of ARI and ACI may be substantially different. The ACI accounts for 64 to 87 % of the aerosol-forcing-sensitivity and is the main source of differences between the ESMs. The ACI can be further decomposed into a cloud-amount term (which depends linearly on cloud fraction) and a cloud-albedo term (which is independent of cloud fraction, to the first order). The large uncertainties of cloud-amount term are responsible for the aerosol-forcing-sensitivity differences and further the anomalous cooling differences among ESMs. The metrics used here therefore provide a simple way of assessing the physical mechanisms contributing to anomalous twentieth century cooling in any given ESM, which may benefit future model developments.

Jie Zhang et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2021-570', Anonymous Referee #1, 10 Sep 2021
  • RC2: 'Comment on acp-2021-570', Anonymous Referee #2, 22 Sep 2021

Jie Zhang et al.

Jie Zhang et al.


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Short summary
The CMIP6 ESMs systematically underestimate TAS anomalies in the NH midlatitudes, especially from 1960 to 1990. The anomalous cooling is concurrent in time and space with anthropogenic SO2 emissions. The overestimated aerosol concentrations and aerosol-forcing-sensitivity (AFS) are responsible for the spurious drop in TAS. Considering the crucial role of cloud properties on the inter-model spread in AFS, the aerosol-cloud interactions should be a focus in development of aerosol schemes.